Caracterização elétrica e microestrutural de vitrocerâmicas condutoras por íon lítio com estrutura NASICON

Abstract

This doctoral thesis presents results of the chemical, thermal, structural, microstructural and electrical characterization of glass and glass-ceramics with Li1,3Al0,3Ti1,7(PO4)3 (LATP), Li1,5Al0,5Ge1,5(PO4)3 (LAGP) and (LixNa1-x)1,5Al0,5Ge1,5(PO4)3 (LNAGP) compositions. All the compositions are shown to present homogeneous volume nucleation (Tgr ≤ 0.6), enabling their microstructures to be controlled through heat treatments, which in this study were single and double. The highly conductive NASICON-type phase was obtained in all the studied compositions. Secondary phases were also observed, albeit with no deleterious effect on the total electrical conductivity. Electrical characterization by impedance spectroscopy, performed at various temperatures below ambient temperature, enabled the contributions of the grain and grain boundary on the total electrical conductivity to be separated. An analysis of the two glass-ceramics, LATP and LAGP, leads to the conclusion that, in both cases, the total electrical resistance of the samples, and hence, their total electrical conductivity, is dominated by the grain boundary. The LATP composition presented higher electrical conductivity at room temperature, but also lower glass forming ability (very low Kgl). A Rietveld refinement of the LATP samples obtained by single heat treatment revealed that the concentration of lithium ions in the unit cell increases as a function of the heat treatment temperature. The electrical conductivity results indicate a relationship between this property and the thickness and volume fraction of grain boundaries, since the latter decrease with heat treatment temperature (in TTS case) and increase with nucleation time (in the case of TTD). The LATP samples obtained by TTD exhibited higher electrical conductivity than those obtained by TTS at the same temperature; however, this effect was not observed in the LAGP samples. The electrical conductivity of the LNAGP samples showed a mixed alkali effect in both the glasses and the corresponding glass-ceramics.